Compound semiconductor materials such as gallium arsenide, indium phosphide, gallium phosphide and mercury cadmium telluride are well known in the electronics industry as finding use in microwave oscillators, semiconductor light-emitting diodes, lasers, IR sensors or the like. In the prior art, these materials are generally prepared by a vapor phase epitaxial (VPE) method of forming at least one active layer on a crystalline substrate. It is known from the past that a compound semiconductor represented by the formula: MQ wherein M is a Group III element and Q is a Group V element can be prepared by the VPE method of reacting a trialkyl compound of element M with a gaseous compound, typically hydride of element Q.
The VPE method is advantageous in the manufacture of gallium arsenide from Ga(CH3)3 and AsH3, for example. Thus, organometallic compounds, especially trialkyl compounds of Group III elements such as trimethylgallium and trimethylindium become of greater interest for the manufacture of semiconductor materials.
The quality of a compound semiconductor obtained by epitaxial growth of an organometallic compound largely depends on the purity of the starting organometallic compound. This is because impurities in the organometallic compound have substantial negative impact on the electrical and optical characteristics of the semiconductor.
In the preparation of organometallic compounds, on the other hand, organic solvents, typically hydrocarbon compounds are used solely for uniform reaction to take place. However, during the reaction, the solvent can be partially decomposed. Among decomposed products, hydrocarbon fragments having a boiling point close to that of the organometallic compound (referred to as “close boiling hydrocarbons”) are entrained on the organometallic compound.
While the current industry demands high purity organometallic compounds, the inclusion of such decomposition products, especially close boiling hydrocarbons necessitates a subsequent purifying step that imposes an extra cost. Sometimes, their separation is difficult.
The close boiling hydrocarbons which cannot be removed from the organometallic compound are regarded quite harmful because they can produce carbon inclusions in any films which are formed from the organometallic compound.
It is known in the art to use a gallium halide and an alkyl aluminum compound as reactants in the preparation of an alkyl gallium compound. See J. Am. Chem. Soc., 84, 3606 (1962). The solvents used in such reaction include saturated hydrocarbons such as pentane, hexane, heptane, octane, nonane, decane and hexadecane, aromatics such as benzene, toluene and xylene (see JP-A 2002-533348), and kerosine, gasoline, and liquid paraffin. The use of these solvents, however, prevents the recovery of high purity alkyl gallium because hydrocarbons produced by side reaction, especially hydrocarbons of 5 to 8 carbon atoms, and benzene are difficult to separate when the alkyl gallium is recovered by distillation.